-LRB- CNN -RRB- Incumbent Goodluck Jonathan phoned former military leader Muhammadu Buhari on Tuesday to concede defeat in Nigeria 's presidential elections , Buhari 's party says .

Jonathan acknowledged the phone call and his defeat in a written statement to his countrymen .

`` I thank all Nigerians once again for the great opportunity I was given to lead this country and assure you that I will continue to do my best at the helm of national affairs until the end of my tenure , '' he said .

The Independent National Electoral Commission is still announcing the final tally in the polls , but early numbers indicate Buhari , now the President-elect , has an overwhelming majority of votes .

Buhari ruled Nigeria from late 1983 until August 1985 after ousting his predecessor in a coup .

His 20-month rule was known for what he described as a `` war on indiscipline , '' a tough regime that some say was marred by human rights abuses .

The 72-year-old retired major general 's experience as a military ruler has variably been viewed as a plus or minus in present-day Nigeria , where the government has been locked in a deadly battle with the militant group Boko Haram .

His campaign has focused on security and ending corruption in Nigeria .

Read more : Who is Nigeria 's Muhammadu Buhari ?

Violent protests after elections Saturday sparked calls for calm from the two main candidates and a warning by the United States and Britain against political interference .

Demonstrators fired gunshots and torched a local electoral office in Nigeria 's oil-rich Rivers state on Sunday as they marched to protest the elections amid claims of vote-rigging and voter intimidation .

After the protests in Rivers , Buhari 's All Progressives Congress demanded the elections there be canceled . `` There 's been so much violence in Rivers state that it 's just not tenable , '' party spokesman Lai Mohammed said .

But the People 's Democratic Party disputed the accusation , saying the election was `` credible and the result reflects the overwhelming wish of the people of Rivers state to support President Goodluck Jonathan . ''

Both candidates took to social media to call for calm .

`` I want to urge all Nigerians to also wait patiently for the Independent National Electoral Commission , INEC , to collate and announce results , '' Jonathan said on his Facebook account .

`` Fellow Nigerians , I urge you to exercise patience and vigilance as we wait for all results to be announced , '' Buhari said on Twitter .

Jonathan and Buhari last week issued a pledge reaffirming their commitment to `` free , fair and credible elections '' after their signing of the Abuja Accord in January .

In his statement Tuesday , Jonathan said ; `` I promised the country free and fair elections . I have kept my word . ''

He advised anyone upset with the results to follow due process and stay away from further violence .

`` As I have always affirmed , nobody 's ambition is worth the blood of any Nigerian . The unity , stability and progress of our dear country is more important than anything else , '' he said .

Read more : Democracy was the real winner

More than 800 people were killed in post-election violence across Nigeria 's north in 2011 after charges that those elections were illegitimate .

Nigeria 's vote had been scheduled for February 14 , but on February 7 , Nigeria 's election commission announced it would be postponed for six weeks because of security concerns , with the military needing more time to secure areas controlled by Boko Haram . The controversial decision was unpopular among many Nigerians and led to widespread protests .

Jonathan has been criticized for not doing enough to combat Boko Haram , which is waging a campaign of terror aimed at instituting a stricter version of Sharia law in Nigeria .

On Saturday , residents in the northeastern state of Gombe said at least 11 people were killed and two more injured in attacks at polling stations , apparently by Boko Haram extremists .

CNN 's Christian Purefoy reported from Lagos , Nigeria , while CNN 's Susannah Cullinane and Stephanie Busari wrote from London .

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Incumbent President Goodluck Jonathan acknowledges defeat , says he delivered on promise of fair elections

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Muhammadu Buhari 's party says Jonathan called to concede even before final results are announced

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Buhari is a 72-year-old retired major general who ruled in Nigeria in the 1980s